1. Field of the Invention
The present invention relates to a die casting machine using vacuum die casting, that is, die casting in a state with pressure in the cavity reduced.
2. Description of the Related Art
A die casting machine is provided with a pair of dies, a fixed die plate and a movable die plate for holding these dies, a clamping apparatus for clamping the dies, an injection apparatus for injecting molten metal into a cavity formed between the dies, a molten metal supplying apparatus for supplying the molten metal to the injection apparatus, and so on. In such a die casting machine, a die casting is obtained by clamping the dies, supplying molten metal into a sleeve of the injection apparatus, injecting the molten metal into the cavity, and filling the cavity with the molten metal.
One of the causes of uneven quality of die castings is the inclusion of gas in the die castings. That is, molten metal injected into the cavity and filled in the cavity at a high speed and under a high pressure forms a turbulent flow in the sleeve and the cavity. Due to this, gas such as air or vaporized parting agent is mixed into the molten metal.
In order to overcome the above problem, there is known the vacuum casting method for decreasing the inclusion of gas and reducing unevenness of die castings caused by the inclusion.
In a die casting machine using the vacuum casting method, as disclosed for example in U.S. Pat. No. 2,785,448, the inclusion of gas into the molten metal is suppressed by injecting the molten metal into the cavity and filling the cavity with the molten metal in a state with the pressure reduced by a vacuum pump.
In the above die casting machine using vacuum casting, in order to cast a product with a high strength and high quality, it is required to be able to create a higher vacuum in the cavity and maintain the vacuum state.
If the cavity is not made a high vacuum, it is difficult to obtain enough of an effect by the vacuum die casting, because gas becomes included in the casting and distortion or a deformation of the product easily occurs when annealing or otherwise heat treating the product after casting.
In order to cast a product with a higher strength and a higher quality, specifically, it is desired to reduce the pressure in the cavity to several tens of Torr.
Further, from the viewpoint of improving the productivity of a die casting machine, it is required to shorten the time required for evacuation by the vacuum pump as far as possible.